1. Field of the Invention
The present invention relates to a bonding structure for a metal plate and a piezoelectric body, such as a piezoelectric vibration plate for use in a driver of a piezoelectric microblower or other suitable piezoelectric device, and a bonding method therefor.
2. Description of the Related Art
Piezoelectric microblowers have been known as blowers for effectively releasing heat produced in housings of portable electronic devices, or as blowers for supplying oxygen required for the generation of electricity in fuel cells (see, for example, WO2008/069266). The piezoelectric microblower refers to a kind of pump using a vibration plate which produces a bending vibration by applying a voltage, and has the advantages of having a simple structure, being able to be configured to have a small and thin size, and providing low power consumption.
FIG. 1 shows an example of a vibration plate for use in a piezoelectric microblower. In FIG. 1, a piezoelectric body (piezoelectric ceramic plate) 2 including electrodes 3, 4 on front and back sides thereof is bonded onto a metallic diaphragm 1. One electrode 3 and the diaphragm 1 are electrically connected to each other. When a predetermined alternating-current voltage is applied between the diaphragm 1 and the electrode 4 of the piezoelectric body 2, the entire vibration plate produces a bending vibration, thereby enabling it to pump the air. It is to be noted that the vibration plate is not limited to the piezoelectric body 2 directly bonded onto the diaphragm 1, and may have various configurations, such as a vibration plate which has another metal plate bonded onto the diaphragm 1 and the piezoelectric body 2 bonded thereon, and a vibration plate which has a piezoelectric body 2 bonded onto both of the front and back sides of the diaphragm 1.
FIGS. 2A to 2C show examples of a bonding structure between the electrode 3 of the piezoelectric body 2 and the metal plate 1 (diaphragm).
FIG. 2A is an example of using an adhesive 5 including no electrically conductive aid, which is intended to provide electrical conduction through contact (ohmic contact) between the electrode 3 and the metal plate 1 by reducing the adhesive thickness between the piezoelectric body 2 and the metal plate 1 to the greatest extent possible.
In FIG. 2B, carbon spheres 6 are added as an electrically conductive aid to the adhesive 5, thereby providing conductivity through the carbon spheres 6. The carbon spheres 6 have a diameter, for example, on the order of 20 μm, and the electrode 3 is not directly connected to the metallic plate 1.
In FIG. 2C, carbon black 7 with an average particle size of several tens of nm is added as an electrically conductive aid to the adhesive 5, thereby providing conductivity through the carbon black 7 in addition to conductivity through contact between the electrode 3 of the piezoelectric body 2 and the metal plate 1.
In the case of using the adhesive 5 including no electrically conductive aid as shown in FIG. 2A, the problem of an increase in resistance value (a decrease in conductivity) is caused when the adhesive is swollen in a humidity test after the bonding. In the case of using the adhesive 5 with the carbon spheres 6 added thereto as in FIG. 2B, the piezoelectric body 2 is likely to have cracks caused from the carbon spheres 6, and moreover, the thickness of the adhesive is increased, thereby resulting in the problem of degraded vibration characteristics of the piezoelectric body 2. In addition, the problem of increased resistance value is caused because of the contact with the electrode function as point contact, or because the largest particle functions as a spacer, whereas the smaller-size particles fail to contribute to electrical conduction. In the case of using the carbon black 7 with a minute particle size as in FIG. 2C, the viscosity-thixotropy of the adhesive is affected significantly by the content of the carbon black 7, which has the problem of having an adverse effect on the workability and application property. Furthermore, the problem of an increase in resistance value (a decrease in conductivity) is also caused when the adhesive is swollen in a humidity test after the bonding.
Japanese Patent Application Laid-Open No. 2001-316655 discloses an electrically conductive adhesive which is used for bonding between an active material layer and a collector in an energy storage element. This adhesive is an electrically conductive adhesive in a paste form, which includes carbon powder (for example, carbon black) as an electrically conductive material, a resin as a binding agent, and water as a solvent, in which primary particles of the carbon powder have a weight-average particle size in the range of 5 nm to 100 nm, the amount of the carbon powder falls within the range of 5 weight % to 50 weight % with respect to the total amount of the carbon powder and resin, and the moisture content of the electrically conductive adhesive in the paste form is supposed to fall within the range of 70 weight % to 95 weight %.
FIG. 2D shows an example of a bonding structure using the electrically conductive adhesive disclosed in Japanese Patent Application Laid-Open No. 2001-316655. As in the case of FIG. 2C, the carbon black 7 is added as an electrically conductive aid to the adhesive 5, thereby providing conductivity through the carbon black 7 without direct electrical conduction between the electrode 3 and the metal plate 1. Since a larger amount of carbon black 7 is added than in FIG. 2C, the conductivity is believed to be improved more than in FIG. 2C.
However, when the carbon black 7 having a particle size of 5 nm to 100 nm is contained in the resin in a large amount of weight % to 50 weight %, the viscosity or thixotropy of the adhesive is greatly increased so as to adversely affect the application stability (variations in the amount of application, leveling properties after the application). Therefore, this problem is solved by water (70 weight % to 95 weight %) included in the paste in Japanese Patent Application Laid-Open No. 2001-316655. For this reason, it is not possible to solve the problem described in the case of a reaction system (such as an epoxy resin) which does not use water. In addition, the larger content of the carbon black 7 relatively reduces the amount of resin, thus decreasing the adhesion. Furthermore, the large amount of water included as a solvent provides a porous formation when the adhesive is hardened, thereby resulting in the problem of making the adhesive more likely to be swollen by absorption of water, and thus, lacking long-term reliability.